Field of the Invention
The present invention relates to measurement of the three-dimensional shape of an object to be measured.
Description of the Related Art
There is known a three-dimensional measurement apparatus that projects a slit pattern typified by a space encoding method from a projection unit such as a projector to an object to be measured (to be referred to as a “target object” hereinafter), and obtains the three-dimensional coordinates of the target object by the principle of triangulation based on a position at which an image capturing unit has observed the reflected light. One problem with measurement by this apparatus is that the measurement accuracy of three-dimensional coordinates greatly depends on the material of a target object.
For a target object made of a material such as plastic, the measurement accuracy may degrade or measurement itself may become impossible owing to a phenomenon called subsurface scattering or internal scattering. Such a target object needs to undergo a treatment of, for example, coating in advance the surface of the target object with a white powder or the like. This becomes an obstacle that greatly limits the scope of application of three-dimensional measurement apparatuses.
As a method of avoiding observation of internal scattering, Japanese Patent Laid-Open No. 2008-281399 (Reference 1) discloses a method in which a linear polarization plate is arranged in an optical path to separate surface reflected light and internal scattering light, and the three-dimensional shape of a target object including a semitransparent portion is accurately measured.
As a method of suppressing the effect of internal scattering, Reference 2 proposes a three-dimensional shape measurement method of modulating slit light by a maximum length sequence (MLS) including a high-frequency component to reduce the effect of internal scattering.
Reference 2: Tatsuhiko Furuse, Shinsaku Hiura, Kosuke Sato, “More Accurate 3D Scanning Method by Controlling Subsurface Scattering”, MIRU2008 Meeting on Image Recognition and Understanding
Japanese Patent Laid-Open No. 2006-275529 (Reference 3) discloses a method of preventing the effect, on a peripheral target object, of inter-reflection caused by secondary reflection between objects when projecting a phase shift pattern to three-dimensionally measure a plurality of target objects, though this method is not a countermeasure against the effect of internal scattering. More specifically, Reference 3 discloses a method of performing three-dimensional measurement while avoiding secondary reflection from an adjacent object using a checkerboard pattern mask.
However, the method in Reference 1 separates surface reflected light and internal scattering light using polarized light, so the geometrical relationship between the light source, the light receiving portion, and the measurement target surface needs to satisfy a predetermined condition. It is not always easy to satisfy this condition and perform measurement.
Since the method in Reference 2 modulates a gray code by MLS, for example, as many as 31 types of patterns need to be projected to each bit of the gray code, greatly prolonging the measurement time.
The method of performing three-dimensional measurement using the checkerboard pattern mask in Reference 3 suppresses only the effect of inter-reflection from a non-projection region adjacent to a projection region in the checkerboard pattern. Although this method is effective for an ideal target object for which an inter-reflection generating portion can be limited, it cannot cope with more common inter-reflection, or internal scattering that affects a region near the incident position of incident light on a non-measurement target.